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TPS61194-Q1: Fsw setting difference between SYNC and FSET

Ryuji
Guru 20090 points
Part Number: TPS61194-Q1
Other Parts Discussed in Thread: TPS61194,

Hello,

In our customer switching behavior is difference when they set the Fsw by SYNC and FSET.
Please see the attached file as the switching waveform.
TPS61194 SW current.pdf

In the attached file, the device is in Pulse skip mode(?) when higher input voltage. They set the Fsw as 2MHz but the Inductor is 22uH. I requested that they change the inductor from 22uH to 4.7uH as datasheet table 2 parameters and I didn't get the information of this.
Then I got the question from the customer.


(1)
Is this waveform correct behavior? Is this pulse skip mode operation? ( I called pulse skip mode for this waveform in this post.)
I think that the device did not stable operation due to higher inductance with higher frequency. 

(2)
When they increase input voltage, the device is in pulse skip mode. But the  transition point is difference between SYNC Fsw setting and FSET Fsw setting.
Could you please let me know this reason?

(3)
Are there any recommended ripple current ratio for stable operation?

(4)
Could you please let me know the condition of forced PWM mode?

Best Regards,
Ryuji



Best Regards,
Ryuji

  • 0
    TI__Expert 4565 points

    Hi Ryuji-san,

    Please see my answers for each question below.

    (1)
    Is this waveform correct behavior? Is this pulse skip mode operation? ( I called pulse skip mode for this waveform in this post.)
    I think that the device did not stable operation due to higher inductance with higher frequency. 

    A: It looks inductor value is too big for this high freq and cause interval of PFM mode longer, so current waveform may seem wrong. But waveform itself looks typical boost operation in PFM(or pulse skipping mode) at light load condition. High inductor and high SW freq will actually help reduce current ripple, and boost stability decreases with too high inductor value regardless of current ripple in general

    (2)
    When they increase input voltage, the device is in pulse skip mode. But the  transition point is difference between SYNC Fsw setting and FSET Fsw setting.
    Could you please let me know this reason?

    A: there is a description in datasheet. "External SYNC frequency must be 1.2 to 1.5 times higher than the frequency defined by RFSET resistor. Minimum
    frequency setting with RFSET is 250 kHz to support 300-kHz switching with external clock" I think Fset setting used for this test is making 2MHz which is same freq from SYNC input, and this difference might have changed internal boost compensation causing PFM threshold difference.

    Please check if:

    1. this test was done with same TPS61194 device?

    2. Fset resistor value was correct to use external 2MHz SYNC input?

    3. What if 4.7uH is used?

    4. Any actual issue on Vout?

    (3)
    Are there any recommended ripple current ratio for stable operation?

    A: As mentioned above, higher inductance will lower current ripple by SW in general, but in this case, bigger current ripple happens by PFM mode SW. So, please try using correct size of inductor. If voltage rating of Cout(ceramic caps) is less than 0.5xVout, using more caps will also help to achieve high enough effective capacitance

    (4)
    Could you please let me know the condition of forced PWM mode?

    A: Unfortunately, TPS61194-Q1 doesn't have this mode. SW mode changes automatically with SW freq and load size

    Hope this helps,

  • 0 in reply to Sung Ho Yoon
    Guru 20090 points

    Hello Sung san,

    I appreciate your support.
    I have the additional question.

    (5) Could you please let me know the PFM threshold? Is the device monitored volley of ripple current for PFM?


    My answer about (2) is the following.

    1. this test was done with same TPS61194 device?
    -->Yes. They test was done with same TPS61194 device on the EVM.

    2. Fset resistor value was correct to use external 2MHz SYNC input?
    -->Yes. They change RFSET value when they input external 2MHz. And the device operated 2MHz switching correctly.

    3. What if 4.7uH is used?
    --> I will check.

    4. Any actual issue on Vout?
    --> I will check

    Best Regards,
    Ryuji

  • 0 in reply to Ryuji
    TI__Expert 4565 points
    Hi Ryuji-san,

    Thanks for your updates.
    Please send me an email for PFM threshold calculation. sungho.yoon@ti.com